1. Field of the Invention
This invention relates to the chlorination of a polyolefin in an aqueous suspension system, and more particularly, to a process for producing a chlorinated polyolefin having a fine particle size and an excellent flexibility and thermal stability.
2. Description of the Related Art
Usually, chlorinated polyolefins are industrially produced by chlorinating polyolefin particles under a considerably high temperature condition in an aqueous suspension system. The various chlorinated polyolefins having characteristics according to the intended use, i.e., from crystalline plastic-like products to noncrystalline rubber-like products, can be produced by appropriately selecting the kinds of starting polyolefins, reaction temperatures, and chlorine contents.
To obtain chlorinated polyolefins having excellent characteristics, it is important to sufficiently disperse the starting polyolefin particles in water. When the starting polyolefin particles are insufficiently dispersed, the chlorine contents in the resultant chlorinated polyolefin particles are largely scattered, or only the surface of the particles is excessively chlorinated, and as a result, nonuniform chlorinated polyolefins are formed. Such chlorinated polyolefins have a poor thermal stability and cause coloration and other problems when molding.
To solve the above-mentioned problems, surfactants including, for example, nonionic surfactants such as ethyleneoxide-propyleneoxide block polymer, polyoxyethylene alkylallyl ether, polyoxyethylene alkyl ether, oxyethylene oxypropylene block polymer; and anionic surfactants such as alkylbenzensulfonic acid salts, alkylnaphthalenesulfonic acid salts, dialkylsulfosuccinic acid salts, alkylphosphoric acid salts, alkylsulfate salts, naphthalenesulfonic acid-formalin condensates, polyoxyethylene alkylsulfuric acid ester are generally added to the aqueous suspension polymerization system.
Nevertheless, in the conventional methods, when a surfactant having a high dispersing effect is used, an extensive foaming occurs in the suspension system, a portion of the polyolefin particles is lifted up to the gas phase with foam, and as a result, that portion is excessively chlorinated. On the other hand, when a less foamable surfactant is used, the chlorine contents between the chlorinated polyolefin particles become nonuniform, due to the poor dispersion.
Furthermore, to maintain a good dispersion during the chlorination reaction, divided additions of the surfactant, or the use of a defoaming agent in the case of an excessive foaming, have been proposed in the art. Nevertheless, the good dispersion conditions are difficult to obtain by these methods, and thus these methods are not industrially effective.
Moreover, when chlorinated polyolefins having an excellent flexibility are produced, the chlorination reaction should be carried out at a temperature near the melting point of the starting polyolefin, and therefore, the surfaces of the polyolefin particles are brought to a semi-molten state. Accordingly, polyolefin particles mutually adhere to each other, and tend to cause agglomeration during the reaction. The agglomerated polyolefin particles are nonuniformly chlorinated, and therefore, the resultant chlorinated polyolefin has poor flexibility. The chlorinated polyolefin thus formed has a large and nonuniform particle size, and the acid and the other impurities contained in the agglomerated particles are not completely removed. Thus, when the produced chlorinated polyolefin is molded and processed, the problems arise of, for example, the occurrence of discoloration and a lowering of the thermal stability. Furthermore, when the chlorinated polyolefin is blended with other resin(s), a poor dispersibility is likely to appear and it becomes difficult to produce a composition having excellent characteristics.
Although the above-mentioned agglomeration of the particles can be prevented to some extent, by lowering the reaction temperature, only a chlorinated polyolefin having a poor flexibility can be obtained. Furthermore, although the agglomeration can be prevented by extremely lowering the amount of the polyolefin, this method is economically disadvantageous from the industrial viewpoint.
As method of preventing an agglomeration of the particles, it is known that relatively inactive substances such as the powder particles of polyvinylchloride (PVC) can be added (see JP-B-38-3573) or inorganic powder particles such as silicon oxide, talc, clay can be added (see U.S. Patent No. 3,454,544). Nevertheless, those methods are not effective unless a large amount of the above-mentioned additives is added, and therefore, a large amount of these additives remains in the product, and thus the use thereof is naturally limited.
In view of the above-mentioned situation in the prior art, various attempts have been made to prevent the mutual agglomeration of the polyolefin particles by adding, as an agglomeration preventing agent, dispersing agents such as polyvinyl alcohol, carboxymethyl cellulose, starch, gelatin; nonionic surfactants such as ethyleneoxide-propyleneoxide block polymer, polyoxyethylene alkylallyI ether, polyoxyethylene alkyl ether, oxyethylene oxypropylene block polymer; and anionic surfactants such as alkylbenzenesulfonic acid salts, alkylnaphthalenesulfonic acid salts, dialkylsulfosuccinic acid salts, alkylphosphoric acid salts, alkylsulfate salts, naphthalenesulfonic acid-formalin condensates, polyoxyethylene alkylsulfuric acid ester.
Nevertheless, in the method of adding a dispersing agent such as polyvinyl alcohol, the polyvinyl alcohol per se is chlorinated and consumed during the reaction and thus the intended effect is reduced, and furthermore, the above-mentioned chlorinated dispersing agent contaminates the product and causes discoloration due to the thermal deterioration thereof during the molding or processing. Moreover, the method of adding a nonionic surfactant such as ethyleneoxide-propyleneoxide block polymer has problems in that the surfactant tends to be gradually decomposed and consumed by the action of hydrochloric acid and hydrochlorous acid formed as a by-product in the chlorination reaction. Therefore, the agglomeration of the particles generally occurs. Accordingly, in these methods, the dispersing agent is added during the reaction, to compensate the consumed amount of the initial dispersing agent, or a larger amount is initially added. Nevertheless, although the unpreferabe agglomeration can be prevented to some extent by these methods, the effects thereof are still not sufficient, and the thermal stability of the product is remarkably lowered by the addition of a large amount of dispersing agents or surfactants.
As mentioned above, the selection of a dispersing agent of agglomeration preventing agent in the production of chlorinated polyolefin is a very important problem, and the desired noncrystalline (or amorphous) chlorinated polyolefin having an excellent flexibility and good thermal stability has not been obtained as yet.